ROS2 cross compile colcon build: Could NOT find Python3 (missing: Python3_NumPy_INCLUDE_DIRS NumPy) (found version "3.8.10")

Question

cross compile question

Host: ubuntu20.04 x86_64

target: ubuntu20.04 aarch64

toolchain: gcc-linaro-7.5.0-2019.12-x86_64_aarch64-linux-gnu

sysroot: sysroot-glibc-linaro-2.25-2019.12-aarch64-linux-gnu

Before I cross compile the python

when I type the command, cmake output the following error information:

$ colcon build --packages-select cv_bridge --build-base crossbuild --install crossinstall --cmake-args -DCMAKE_TOOLCHAIN_FILE=/path/to/aarch64-linux-gnu.cmake
Starting >>> cv_bridge
--- stderr: cv_bridge                         
CMake Error at /usr/share/cmake-3.16/Modules/FindPackageHandleStandardArgs.cmake:146 (message):
  Could NOT find Python3 (missing: Python3_LIBRARIES Python3_INCLUDE_DIRS
  Python3_NumPy_INCLUDE_DIRS Development NumPy) (found version "3.8.10")
Call Stack (most recent call first):
  /usr/share/cmake-3.16/Modules/FindPackageHandleStandardArgs.cmake:393 (_FPHSA_FAILURE_MESSAGE)
  /usr/share/cmake-3.16/Modules/FindPython/Support.cmake:2214 (find_package_handle_standard_args)
  /usr/share/cmake-3.16/Modules/FindPython3.cmake:300 (include)
  CMakeLists.txt:21 (find_package)


---
Failed   <<< cv_bridge [1.98s, exited with code 1]

the key words are that "missing: Python3_LIBRARIES Python3_INCLUDE_DIRS Python3_NumPy_INCLUDE_DIRS Development NumPy"

So I cross compile the python

But throw the error:

Could NOT find Python3 (missing: Python3_NumPy_INCLUDE_DIRS NumPy) (found
  version "3.8.10")

What I have tried

As a noob in CMake, what I can understand is that I have tried is the lack of the numpy library, so I tried to install numpy using crossenv and pip. However, there were too many errors reported, so I gave up on this approach.

Moreover, I also tried copying the numpy* files from the python/site-packages directory on the aarch64 architecture to the cross-compiled python/site-packages directory, but it still didn't solve the problem. I guess even if I manage to install numpy using this method, the final installation location would not be related to the cross-compiled python.

I am hoping someone can answer this question, and I am extremely grateful.

Answer 1

I assume you are using virtualenv or conda environment for cross-compiling, so you have to be more specific about what python binary to use:

colcon build --symlink-install   --cmake-args -DPython3_FIND_VIRTUALENV=ONLY

Or you can directly specify what version you want to use for your colcon build:

colcon build --symlink-install \
  -DPython3_EXECUTABLE:INTERNAL=$CONDA_PREFIX/bin/python3 \
  -DPython3_FIND_STRATEGY=LOCATION -DPython3_FIND_UNVERSIONED_NAMES=FIRST

All other flags are here: https://cmake.org/cmake/help/latest/module/FindPython3.html.

I also assume you have installed numpy in that python env.

Pro tip: you can save these flags in your colcon.pkg file.